In very many industrial applications it is necessary to separate two immiscible fluids of different densities, with one of the fluids being dispersed in the other in the form of globules which may be very small in size.
The proposed invention relates more particularly to purifying reinjection or waste water on offshore oil platforms. In this particular case, droplets of oil need to be extracted from a main flow of water. This "de-oiling" operation must be performed by an apparatus or a set of apparatuses capable of combining as far as possible four essential qualities:
A first quality is separation efficiency. The rate at which the apparatus purifies waste water must be as high as possible. This efficiency is conventionally measured by means of the apparatus cutoff diameter i.e. the globule diameter beyond which all oil globules are removed from the flow of water. Depending on the values of this cutoff diameter, the purification may need to be performed in one, two, or even three stages using a corresponding number of different types of separator connected in series.
A second quality is compactness. Given the very high cost of each "shipped ton" on a platform, platform operators look for apparatuses whose weight and volume are as small as possible. The compactness of an apparatus is characterized, above all, by the length of time the mixture remains in the apparatus: the shorter this length of time, the greater the extent to which volume and weight are reduced.
A third quality is flow rate flexibility. In operation, the flow rate of water to be treated may vary by as much as -50% to -100% from the nominal flow rate, for example over period of a few minutes. It is thus important for the separators to be able to treat such fluctuating flows without losing de-oiling efficiency. In addition, users are looking for apparatuses having as high a flow rate as possible per unit so as to be able to perform treatment with a minimum number of apparatuses.
The fourth quality is low energy consumption. Energy consumption is not a crucial problem on an operating platform, but the available pressures are often limited to a few bars. However, it may be considered that an apparatus consumes energy in the form of head loss, thereby causing zones of intense hydraulic shear to appear which split up the drops of oil into droplets which are so small as to become unseparable. Roughly speaking, the lower the head loss required to obtain separation, the greater the efficiency of the apparatus.
More or less similar qualities are desirable for other operations such as de-watering crude oil and degassing liquids, and more generally for any industrial operation which makes use either of hYdrocyclones or else of centrifuges.
In conventional hydrocyclones used for de-oiling, an intense acceleration field is created by rotating fluid which enters a fixed wall separation chamber tangentially in order to establish a free vortex type of flow. Uhder the effect of centrifugal force, oil concentrates on an axial core and is removed via a special outlet. The intensity of this force is at a maximum at the interface between the core and the surrounding liquid.
In centrifuges which have been developed for de-watering crude oil, the fluid is set into rotation by rotating the walls of the separation chamber by means of an external power source (a motor). These apparatuses set up a bulk rotary flow in which the speed of rotation and centrifugal force are very low in the vicinity of the axis. Uhder these conditions it is impossible to concentrate the light phase sufficiently to be able to extract the heavy phase easily. In contrast, in a free vortex type of flow, the very high speeds of rotation in the vicinity of the axis ensure that a stable core of light phase is formed.
Further, French Pat. No. 80 07 244 published under the No. 2 478 489 and its corresponding U.S. Pat. No. 4,443,331 describe a separator used in the paper pulp industry. In this apparatus, the walls of the separation chamber are rotated and a free vortex type of flow is set up in its inside volume, with the radius of the outlet opening for the majority liquid being smaller than the radius of the inlet opening. Inlet acceleration guides cause the inlet liquid to rotate and move radially away from the axis of the separator up to the inlet opening radius. Their downstream portions constitute injection blades which cause the inlet liquid to move at constant radius and which are inclined to the axis in such a manner as to give said liquid, when it leaves the blades to enter the separation chamber, an absolute circumferential speed in the same direction as and greater than the absolute circumferential speed of the rotating side wall of said chamber. As a result, friction between said liquid and said wall then sets up a radial gradient in the circumferential speed of the liquid in the vicinity of the wall. This gradient provides stirring. Said French patent indicates that in other separators "the absence of any stirring" leads to the drawback that the "fibrous constituents" of the inlet liquid "tend to associate very rapidly in a coherent network which . . . prevents any displacement within the fluid".
Other such separators are the centrifuges described in French Pat. No. 2 091 170 and the corresponding U.S. Pat. No. 3,862,714 (Broadway).
The aim of the present invention is to obtain improved separation efficiency over prior art separators in a simple manner.
Another aim is to simultaneously improve compactness, flow rate flexibility, and energy consumption of a separator, in particular for the case where a dispersed light fluid such as oil or air is to be separated from a main liquid such as water.